Apple has silently updated the AirPods firmware from 3.3.1 to 3.5.1 earlier today. The AirPods automatically update its firmware when its inside the AirPods Case, charging, and is connected to a nearby iOS device. The new firmware doesn’t seem to have any user-facing features.
When the update takes place, there’s no on-screen alert or notification, meaning AirPods automatically update. However, users can verify the update by heading to Settings > General > About > AirPods to check the new firmware version.
If your AirPods haven’t been updated to 3.5.1 yet, there’s nothing you can do other than put the AirPods back in the case and wait for it to update on its own. That being said, there doesn’t seem to be any major changes and it appears that this is a ‘bug fix and stability improvements’ update.